FIG. 1 shows the schematic construction of a conventional induction-coupled plasma processing apparatus 1.
Referring to FIG. 1, the plasma processing apparatus 1 includes a processing vessel 2 of a quartz dome evacuated by an evacuation line 2A, and there is provided a stage 3 in a process space 2B defined by the processing vessel 2 such that the stage 2 is rotated by a rotating mechanism 3A. Further, a substrate 4 is held on the stage 3. Further, an inert gas such as Ar and a process gas such as oxygen or nitrogen are supplied to the process space 2B via a process gas supply line 2C. Further, there is provided a coil 5 around the top part of the processing vessel 2 at the outside thereof, and high-density plasma 2D is inducted at the top part of the process space 2B by driving the coil 5 by a d.c. power source.
In the plasma processing apparatus 1 of FIG. 1, the radicals of the process gas formed with the high-density plasma 2D reach the surface of the substrate 4 and the substrate processing such as oxidation or nitridation is achieved.
In such a conventional induction-coupled plasma processing apparatus 1, on the other hand, there exists a drawback in that the high-density plasma 2D is localized at the top part of the processing vessel and there appears an extremely non-uniform distribution in the radicals that are formed with the plasma. Particularly, the non-uniformity of the radical concentration in the radial direction of the substrate is not resolved even when the stage 3 is rotated by the rotating mechanism 3A.
Thus, in the conventional induction-coupled plasma processing apparatus 1, the plasma processing apparatus was designed such that the substrate 4 is separated from the region in which the high-density plasma 2D is formed with a large distance for realizing as uniform radical concentration distribution as possible at the surface of the substrate 4. As a result of such a construction, on the other hand, the overall size of the substrate processing apparatus 1 is increased inevitably. Further, the amount of the radicals reaching the substrate 4 is reduced. These problems become particularly serious in the technology of current trend of processing a large-diameter substrate.
On the other hand, there is a proposal of a microwave plasma processing apparatus that uses high-density plasma induced, not by an induction magnetic field but by a microwave electric field. For example, there is proposed a plasma processing apparatus that uses a planar antenna (radial line slot antenna) having a large number of slots arranged so as to produce a uniform microwave, for emitting a microwave into a processing vessel. In this apparatus, the microwave electric field thus induced is used to excite plasma by causing ionization in the gas in the vacuum vessel. Reference should be made to Japanese Laid-Open Patent Application 9-63793. By using the microwave plasma excited according to such a process, it becomes possible to realize a high-plasma density over a wide area right underneath the antenna, and uniform plasma processing becomes possible with short time period. Further, the microwave plasma thus excited has an advantageous feature of low electron temperature as a result of excitation of the plasma by using a microwave, and it becomes possible to avoid the problem of damages or metal contamination caused in the substrate. Further, it becomes possible to excite uniform plasma over a substrate of large area, and thus, the plasma processing apparatus can easily handle the fabrication of semiconductor devices on a large-diameter semiconductor wafer or fabrication of large flat panel display devices.
FIG. 2 shows the construction of a microwave plasma processing apparatus 10 that uses such a radial line slot antenna as proposed before by the inventor of the present invention.
Referring to FIG. 2, the microwave plasma processing apparatus 10 includes a processing chamber 11 evacuated at a plurality of evacuation ports 11a, and there is provided a stage 13 inside the processing chamber 11 for supporting a substrate 12 to be processed. In order to achieve uniform evacuation of the processing chamber 11, there is provided a ring-shaped space 11A around the stage 13, and the processing chamber 11 is evacuated uniformly via the space 11A and further via the evacuation ports 11a by arranging the evacuation ports 11a communicating with the space 11A in axial symmetry with respect to the substrate.
On the processing chamber 11, there is provided a plate-like shower plate 14 formed of a low-loss dielectric such as Al2O3 or SiO2 as a part of the outer wall of the processing chamber 11 at a location facing the substrate 12 held on the stage 13, wherein the shower plate 14 is provided via a seal ring not illustrated and includes a number of apertures 14A. Further, a cover plate 15 also of a low-loss dielectric such as Al2O3 or SiO2 is provided at the outer side of the shower plate 14 via another seal ring not illustrated.
The shower plate 14 is provided with a gas passage 14B at a top surface thereof, and each of the apertures 14A are provided so as to communicate with the gas passage 14B. Further, there is provided a gas supply passage 14C in the interior of the shower plate 14 in communication with a gas supply port 11p provided at an outer wall of the processing vessel 11. Thus, the plasma-excitation gas such as Ar or Kr supplied to the gas supply port 11p is forwarded to the apertures 11A via the supply passage 14C and further via the passage 14B and is released to the process space 11B right underneath the shower plate 14 inside the processing vessel 11 from the foregoing apertures 14A.
On the processing vessel 11, there is further provided a radial line slot antenna 20 at the outer side of the cover plate 15 with a separation of 4-5 mm from the cover plate 15. The radial line slot antenna 20 is connected to an external microwave source (not illustrated) via a coaxial waveguide 21 and causes excitation of the plasma-excitation gas released into the process space 11B by the microwave from the microwave source. It should be noted that the cover plate 15 and the radiation surface of the radial line slot antenna are contacted closely, and there is provided a cooling block 19 on the antenna 20 for cooling the antenna. The cooling block 19 includes a cooling water passage 19A.
The radial line slot antenna 20 is formed of a flat, disk-shaped antenna body 17 connected to an outer waveguide tube 21A of the coaxial waveguide 21 and a radiation plate 16 provided at the opening of the antenna body 17, wherein the radiation plate 16 is formed with a number of slots and a retardation plate of a dielectric plate having a constant thickness is interposed between the antenna body 17 and the radiation plate 16.
In the radial line slot antenna 20 having such a construction, the microwave fed thereto from the coaxial waveguide 21 propagates along a path between the disk-shaped antenna body 17 and the radiation plate 16 in the radial direction, wherein the microwave thus propagating undergoes compression of wavelength as a result of the existence of the retardation plate 18. Thus, by forming the slots concentrically in correspondence to the wavelength of the microwave thus propagating in the radial direction, and by forming the slots so as to form a perpendicular angle with each other, it becomes possible to emit a plane wave having a circular polarization from the radial line slot antenna 20 in the direction substantially perpendicular to the radiation plate 16.
By using such a radial line slot antenna 20, there is formed uniform high-density plasma in the process space 11B right underneath the shower plate 14. The high-density plasma thus formed has a feature of low electron temperature and the occurrence of damages in the substrate 12 to be processed is avoided. Further, there occurs no metal contamination caused by sputtering of the chamber wall of the processing vessel 11.
Thus, by supplying a process gas, such as an O2 gas, an NH3 gas, or a mixed gas of an N2 gas and an H2 gas, to the gas inlet port 11p of the substrate processing apparatus 10 of FIG. 2 in addition to the plasma-excitation gas such as Ar or Kr, there is caused an excitation of active species such as atomic state oxygen O* or hydrogen nitride radicals NH* in the process space 11B by the high-density plasma, and it becomes possible to conduct oxidation processing, nitridation processing or oxynitridation processing on the surface of the substrate 12.
Further, there is proposed a substrate processing apparatus 10A shown in FIG. 3 having a construction similar to the substrate processing apparatus 10 of FIG. 2 except that there is provided a lower shower plate 31 at the lower side of the shower plate 14. The lower shower plate 31 is provided with a process gas passage 31A communicating with a process gas inlet port 11r formed at the surface of the processing vessel 1 and a large number of process gas inlet nozzle openings 31B are formed in communication with the process gas passage 31A. Further, the lower shower plate 31 is provided with large apertures for passing the process gas radicals formed in the space 11B.
Thus, in the substrate processing apparatus 10A of FIG. 3, there is defined another process space 11C underneath the lower shower plate 31. By forming the lower shower plate 31 by a conductive material such as a stainless steel having a passivation surface by aluminum oxide (Al2O3) in such an apparatus, it becomes possible to block the penetration of microwave to the process space 11C. Thereby, the excitation of plasma is limited in the process space 11B right underneath the upper shower plate 14, and the radicals Kr* of Kr or Ar* of Ar penetrate into the process space 11C through the large apertures formed in the shower plate 31 after excitation in the space 11B. The radicals Kr* or Ar* thus penetrated into the process space 11C cause activation of the process gas released from the nozzle apertures 31B, and the processing of the substrate 12 is achieved by the process gas radicals thus activated.
In the substrate processing apparatus 10A of FIG. 3, it should be noted that the microwave is expelled from the process space 11C by forming the lower shower plate 31 by a conductive material, and the damaging of the substrate by microwave is avoided.
In the substrate processing apparatus 10A of FIG. 3, it is also possible to conduct a plasma CVD process by introducing a CVD source gas from the lower shower plate 31. Further, it is possible to conduct a dry etching process by introducing a dry etching gas from the lower shower plate 31 and applying a high-frequency bias to the stage 13.
Thus, in the substrate processing apparatus of FIG. 2 of FIG. 3, Kr radicals (Kr*) of intermediate excitation state having an energy of about 10 eV are excited at the time of conducting an oxidation processing, by introducing a Kr gas and an oxygen gas into the process space 11B. The Kr radicals thus excited cause efficient excitation of atomic state oxygen O* according to the reactionO2→O*+O*,while the atomic state oxygen O* thus excited cause the desired oxidation of the surface of the substrate 12.
In the case of conducting a nitridation processing of the substrate 12, a Kr gas and an ammonia gas, or a Kr gas and a nitrogen gas and a hydrogen gas are introduced. In this case, the excited Kr radicals (Kr*) or Ar radicals (Ar*) cause the excitation of hydrogen nitride radicals NH* according to the reactionNH3→NH*+2H*+e−,orN2+H2→NH*+NH*,wherein the hydrogen nitride radicals thus excited cause the desired nitridation processing of the substrate of the surface 12.
Meanwhile, there are cases in which it is preferable to use atomic state nitrogen (N*), free from hydrogen and having a strong nitriding power, at the time of the nitridation processing of the substrate. The atomic state nitrogen N* are formed according to the reactionN2→N*+N*,wherein it should be noted that such a reaction requires the energy of 23-25 eV. This means that it is not possible to excite the atomic state nitrogen N* according to the foregoing reaction, as long as Kr or Ar plasma is used. As noted previously, the energy of the Kr radicals or Ar radicals obtained by the Kr or Ar plasma is merely in the order of 10 eV.
Thus, even when there is made an attempt to supply a nitrogen gas in the substrate processing apparatus of FIG. 2 or FIG. 3 in place of the Kr gas or the Ar gas, merely the reactionN2→N2++e−,is obtained, and there is caused no desired atomic state oxygen N*.
FIG. 4 shows the relationship between the state density of the Kr plasma and the excitation energy of the atomic state nitrogen N*, hydrogen nitride radicals NH* and nitrogen atoms N2+.
Referring to FIG. 4, it can be seen that the state density of the Kr plasma is large at the low energy side, while the state density shows a rapid decrease with increase of the energy. Such a plasma cannot achieve efficient excitation of the desired nitrogen radicals.